A variable valve system for an internal combustion engine capable of changing the opening and closing timings and the maximum lift amount of an engine valve, is disclosed in JP 2002-155716. The valve system comprises a varying mechanism for controlling the valve lift amount of an intake valve put into an opening operation by a swing cam swingably supported on a drive shaft, and a drive mechanism having an electric motor for rotationally driving a control shaft of a control mechanism for controlling the operating position of the varying mechanism. The electric motor of the valve system is disposed at a rear end portion of a cylinder head with a plate therebetween and substantially in parallel to the control shaft. The drive shaft of the electric motor is disposed substantially in parallel to the drive shaft which is rotatably supported on the cylinder head and which is rotationally driven by the crankshaft.
The electric motor according to the above-mentioned Japanese reference is disposed on the exterior of the cylinder head and exposed to the outside air. Accordingly, the motor, is cooled by a process in which the heat generated by the operation thereof is released into the outside air. This arrangement ensures that accurate operations of the electric motor are secured, and the durability of the electric motor is enhanced. Meanwhile, in an internal combustion engine mounted on a vehicle, when it is intended to promote the cooling of the electric motor by utilizing the running airflow for the purpose of enhancing the performance of cooling by heat radiation, it is necessary to ensure that the collision of the running airflow on the electric motor is not hampered by the cylinder head itself or members disposed in the vicinity of the cylinder head. The limitation restricts the layout of the electric motor and makes it difficult to achieve a compact layout of the electric motor in relation to the cylinder head. When the electric motor is disposed at a tip end portion, in the cylinder axis direction, of a head cover connected to the cylinder head, the valve system comprising the electric motor is enlarged in size in the cylinder axis direction and, hence, the internal combustion engine comprising the valve system is enlarged in size in the cylinder axis direction.